Methodologies employed in the production of systematic reviews used to inform policy must be robust. In formulating the recent World Health Organisation (WHO) Environmental Noise Guidelines for the European Region, seven systematic reviews of evidence were commissioned to assess the relationship between environmental noise exposure and a range of health outcomes, six of which were nonauditory. Within the methodological guidance document devised for these reviews, inclusion and exclusion criteria for individual studies and existing reviews were applied in accordance with the Population-Exposure-Comparator-Outcome-Study (PECOS) framework for the evaluation of evidence. Specific criteria were defined for "populations" and source-specific "exposure", but no criteria were defined for the treatment of potential "effect modifiers". Furthermore, no criteria were set for the treatment of combined exposures. Employing a custom-designed assessment matrix, we assess the treatment of potential effect modifiers in the formulation of the aforementioned systematic reviews, all published in a Special Issue of the International Journal of Environmental Research and Public Health (IJERPH), titled "WHO Noise and Health Evidence Reviews". We identify substantial methodological variation in their treatment and propose the differentiation of "moderators" and "mediators" from "confounders" as the basis for criteria development-including combined exposures-for future systematic reviews.
Keywords: assessment; confounders; effect modification; health; mediators; methodology; moderators; outcomes; systematic reviews; wellbeing.